[TeMPOraL]

But it runs over your Intranet and so is generally as secure for outside access as your VPN is, which presumably gets more attention.

Also, Slack is a much bigger and juicier target for attack.

Ultimately it's a tradeoff. You may need to pay more attention to security in-house, but in exchange you're no longer tying the critical aspects of your business to some random SV web startup that is here today, but may get acquihired tomorrow and kill the product overnight.